A piezoelectric filter using a thickness sliding oscillation or the like of a piezoelectric substrate is generally arranged such that there is housed, in a container, an element comprising a piezoelectric substrate of crystal or the like provided on one surface thereof with a pair of input/output oscillating electrodes (split electrodes) with a predetermined gap provided, and on the other surface thereof with a common electrode opposite to the pair of input/output oscillating electrodes.
Recently, such a piezoelectric filter is required to be made in a small size particularly for a portable-type communication device or the like. Further, to simplify the mounting process, it is required that the element is not only made in a small size, but also housed in a package of the surface mounting type. In such a structure in which the element in a small size is housed in a package of the surface mounting type, however, a leakage current is readily generated due to an electromagnetic coupling between the input/output oscillating electrodes. This is remarkably disadvantageous in that the assured attenuation amount is lowered.
To overcome the problem above-mentioned, there has been proposed a piezoelectric filter which comprises (i) a surface mounting-type package body made of an insulator in which a layer-like shield electrode uniform in grounding potential is formed on a flat portion and (ii) a piezoelectric filter element incorporated in and supported and by the package body, the piezoelectric filter element having a pair of input/output oscillating electrodes arranged to be opposite to the flat layer-like shield electrode, and in which the pair of input/output oscillating electrodes and the shield electrode are opposite to each other with a small distance provided therebetween (For example, Japanese Patent Laid-Open Publications No. 6-29776 and 6-85599 and U.S. Pat. No. 5,382,929).
According to the arrangement above-mentioned, as seen in each of the publications above-mentioned, the assured attenuation amount is increased by reducing the distance between the input/output oscillating electrodes and the shield electrode disposed at the package body. However, when the distance is reduced down to a certain limit value, the assured attenuation amount reaches the peak. As schematically shown in FIG. 10, when the distance is further reduced, floating capacitances C are generated between each of a pair of input/output oscillating electrodes 71, 72 and a shield electrode 73. Accordingly, a leakage current iL is generated between the pair of input/output oscillating electrodes 71, 72, causing the assured attenuation amount to be gradually reduced.
More specifically, even though a flat shield electrode is disposed opposite to a pair of input/output oscillating electrodes with a predetermined gap provided therebetween, the assured attenuation amount is limited to the peak value determined based on the distance between the shield electrode and the pair of input/output oscillating electrodes, and cannot be increased more than this peak value.
In view of the foregoing, the present invention is proposed with the object of providing a piezoelectric filter of the surface mounting type so arranged as to increase the assured attenuation amount as compared with the conventional piezoelectric filter.